One-piece plastic covers made of thermoplastic sheets are advantageous for providing attractive and serviceable covers for books, and with the important savings in the cost of binding the books. With the cover made in one piece, there is a problem in providing the necessary stiffness for the front and rear covers and at the same time to have sufficient flexibility for the hinge lines of the cover.
Greater economy is possible if commercial sheets of plastic are used in the manufacture of the covers; and various methods have been used for the purpose. This invention provides further improvements in the making of plastic covers. Cross-linking of the plastic to a limited degree to improve the elasticity of the hinge portions of the cover may be used to increase the useful life of the cover. More extensive cross-linking is used to add stiffness to the parts of the cover that are the front and rear panels of the book cover.
Some features of the invention relate to laminated cover constructions with the different layers of different susceptibility to cross-linking. If maximum stiffness is desired for the front and rear cover panels, these panels can be substantially thermoset; and the cross-linking diminishes or eliminates migration of plasticizers between the cover panels and adjacent books or other surfaces such as sometimes occurs when printing contacts with a surface of a plastic cover.
Other objects, features and advantages of the invention will appear or be pointed out as the description proceeds.